Here are some of my favorites:

Jill Bates, jbates@esu8.org

HippoCampus

http://www.vernier.com/products/interfaces/lq-mini/ -- Vernier product link

ESU 8 Media Center

eLibrary

http://www.thefutureschannel.com/

http://forensics.rice.edu/

http://ictmagic.wikispaces.com/Science -- lists of recommended science websites

http://learningcenter.nsta.org/default.aspx

http://science360.gov/files/

http://whyfiles.org/ -- the science behind the news

http://www.engineergirl.org/

http://www.ncsu.edu/imse/4/elementaryEd.htm

http://www.schrockguide.net/ -- Kathy Schrock's Guide to Everything

http://school.discoveryeducation.com/schrockguide/?campaign=DE&CFID=2665280&CFTOKEN=97804888

http://www.internet4classrooms.com/sciencefair.htm

http://www.forensicsciencetechnician.org/100-best-websites-for-science-teachers/

http://apps.exploratorium.edu/10cool/index.php?cmd=browse&category=11

http://www.khanacademy.org/

http://www.ck12.org/flexbook/ -- free online textbooks

http://www.exploratorium.edu/

http://www.wordle.net/ -- word clouds

http://www.tagxedo.com/ -- word clouds

http://www.livebinders.com/

http://www.jason.org/public/whatis/start.aspx

http://nsdl.org/ -- National Science Digital Library

http://www.wikispaces.com/

http://www.curriki.org/

http://www.ptable.com/ This site is an interactive Periodic Table

http://scaleofuniverse.com/ Interactive scale of the universe